use strict;
use warnings;

my $flag1 = 0;
my $flag2 = 0;
my @lines;
while (<DATA>) {
    $flag2 = 1 if $flag1 and /A1/;
    $flag1 = 1 if /SUBSCRIBER/;
    push @lines, $_ if $flag1;
    if (/NATIONAL/) {
        print @lines if $flag2;
        $flag1 = 0;
        $flag2 = 0;
        @lines = ();
    }
}

__DATA__
foo
bar
SUBSCRIBER
goo
hoo
nada
NATIONAL

SUBSCRIBER
goo
A1
NATIONAL
junk
junk
junk

##</code><code>##

SUBSCRIBER
goo
A1
NATIONAL

##</code><code>##

use strict;
use warnings;

my $flag = 0;
my @lines;
while (<DATA>) {
    if (/SUBSCRIBER/ .. /NATIONAL/) {
        push @lines, $_;
        $flag = 1 if /A1/;
        if (/NATIONAL/) {
            print @lines if $flag; 
            $flag = 0;
            @lines = ();
        }
    }
}